Staggered Backup Battery Charging System

ABSTRACT

A battery backup system includes a control device including a power sensing device, a discharge circuit, a charging circuit, and a plurality of battery packs. A lower threshold is established representative of a minimum acceptable effective energy capacity. Each battery pack is recharged if its effective energy capacity falls below the lower threshold. Additionally, an upper threshold is established representative of the minimum acceptable effective energy capacity plus a performance margin. In a two battery-pack system, if both battery packs fall below the upper threshold, the battery with the least effective energy capacity is discharged to the minimum acceptable effective energy capacity and then recharged to the battery pack&#39;s maximum energy capacity. In this way, both battery packs are prevented from approaching the minimum acceptable effective energy capacity at the same time. This reduces the size or number of battery packs and reduces their associated cost and volume.

RELATED APPLICATION INFORMATION

This application claims the filing date benefit and is a divisionalapplication of U.S. patent application Ser. No. 10/991,914, filed Nov.18, 2004.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention is related in general to the field of backup-batterysystems. In particular, the invention consists of a system forstaggering the charging of a plurality of backup-battery packs so as toreduce the number or size of the battery packs.

2. Description of the Prior Art

In FIG. 1, a traditional backup-battery system 10 includes a controldevice 12 having a power sensor 14, a charging circuit 16, and one ormore battery packs 18. Each battery pack 18 includes one or morebatteries 20 and has a maximum energy capacity based on the physicalcharacteristics of the batteries. Once discharged below a predetermineddischarge threshold, some battery packs are designed to be recharged totheir maximum energy capacity. However, a battery pack's maximum energycapacity will deteriorate over time due to temperature fluctuations,repetitive charging/discharging, and other chemical reactions. Forexample, a typical battery may lose up to 20% of its maximum energycapacity over its useful life. This decrease is maximum energy capacityis more pronounced in some types of batteries if they are not fullydischarged to their discharge threshold prior to being recharged.

If a battery pack is charged to its current maximum energy capacity, thecharge will also deteriorate over time (“self-discharge”), even when notunder load, due to chemical reactions within the battery. One approachis to provide a continuous charge (“trickle charge”) to the battery packto prevent self-discharge. However, this creates other problems such asdramatically reduced battery life. Accordingly, it is desirable to havea backup-battery system to compensate for self-discharge withoutincurring the reduced battery-life penalty of trickle charging.

The energy available to power devices (“effective energy capacity”) is afunction of the current maximum energy capacity, the current amount ofself-discharge, the current temperature, and the load placed on thebattery pack. This effective energy capacity is often significantly lessthan a battery pack's original maximum energy capacity. Accordingly, abackup-battery system must be designed to provide a sufficient effectiveenergy capacity to power a load, even if the effective energy capacityis significantly lower than a battery pack's original maximum energycapacity.

Another approach is to utilize a battery pack with an initial maximumenergy capacity significantly higher than that needed to power theintended load. However, increasing a backup-battery system's initialmaximum energy capacity is costly and requires a larger volume of spacefor the correspondingly larger battery pack. Additionally, increasingthe size of a battery pack can dramatically increase the weight of thebackup-battery system.

Yet another approach is to utilize multiple battery packs with acombined initial maximum energy capacity significantly greater thannecessary for the design load. Since the packs are in the sameenvironment, they will tend to self-discharge at the same rate,potentially reaching the discharge threshold at approximately the sametime. This solution suffers the same drawbacks as utilizing a singlelarger battery pack, i.e., increased cost, volume, and weight.Therefore, it is desirable to have a system for charging multiplebackup-battery packs that prevents the battery packs from approachingthe discharge threshold at the same time.

In U.S. Pat. No. 6,583,603, Baldwin discloses an apparatus for chargingand discharging battery cells employed as a back-up power supply. Theapparatus partially isolates battery cells from a load bus and powersupply utilizing two control switches arranged in parallel. One of thecontrol switches connects one set of batteries to the load bus at atime. However, while Baldwin discloses charging sets of batteriesindependent of other sets, he does not disclose preventing a pluralityof battery packs from simultaneously approaching a discharge threshold.

In U.S. Pat. No. 6,504,344, Adams et al. disclose a device for managingbattery packs. A controller directs the selective load-testing ofindividual batteries. A battery whose useful capacity has fallen below apredetermined threshold is recharged. However, as with the Baldwinpatent, Adams does not disclose alternating the charging of parallelbattery packs to reduce the amount of excess design capacity.Accordingly, it is desirable to have a backup-battery system capable ofstaggering the charging of multiple battery packs to minimize therequired initial maximum energy capacity.

SUMMARY OF THE INVENTION

The invention disclosed herein utilizes a backup-battery systemincluding a control device having a power sensing device, a chargingcircuit, a plurality of battery packs, and a discharge circuit. Eachbattery pack includes one or more batteries. A lower threshold,representative of a minimum acceptable effective energy capacity, isestablished by the control device. Each battery pack is recharged if itseffective energy capacity falls below the lower threshold. Additionally,an upper threshold is established representative of the minimumacceptable effective energy capacity plus a performance margin.

In a two battery-pack system, if both battery packs fall below the upperthreshold, the battery pack with the least effective energy capacity isrecharged to the battery pack's maximum energy capacity. In this way,both battery packs are prevented from approaching the minimum acceptableeffective energy capacity at the same time. This reduces the size ornumber of battery packs and reduces their associated cost and volume. Inorder to maximize battery life and reduce the deterioration of a batterypack's maximum energy capacity, the battery pack may be discharged tothe minimum acceptable effective energy capacity before being recharged.

The invention can be extended to a backup battery system have three ormore battery packs. If two or more battery packs fall below the upperthreshold, the battery pack with the least effective energy capacity isdischarged and then recharged to maximum energy capacity. Alternatively,the upper threshold may be set so that a battery pack is discharged andrecharged only when all the battery packs fall below the upperthreshold.

Various other purposes and advantages of the invention will become clearfrom its description in the specification that follows and from thenovel features particularly pointed out in the appended claims.Therefore, to the accomplishment of the objectives described above, thisinvention comprises the features hereinafter illustrated in thedrawings, fully described in the detailed description of the preferredembodiments and particularly pointed out in the claims. However, suchdrawings and description disclose just a few of the various ways inwhich the invention may be practiced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating a traditional backup batterysystem including a control device having a power sensor, a chargingcircuit, and a plurality of battery packs, each battery pack includingone or more batteries.

FIG. 2 is a block diagram of a staggered backup-battery charging systemincluding a control device having a power sensor and a lower and anupper threshold, a charging circuit, a discharge circuit, and aplurality of battery packs, each battery pack including one or morebatteries.

FIG. 3 is a block diagram of the control device of FIG. 2, wherein thecontrol device includes a processing device and a memory device, thememory device including memory locations holding the lower and upperthresholds.

FIG. 4 is a block diagram of the control device of FIG. 2, wherein thecontrol device includes a first hardware device for maintaining thelower threshold and a second hardware device for maintaining the upperthreshold.

FIG. 5 is a flow chart illustrating a staggered charging algorithmaccording to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

This invention is based on the idea of using a control device toestablish a lower and an upper threshold of effective energy capacityand to stagger the charging of a plurality of battery packs according tothese thresholds. The invention disclosed herein may be implemented as amethod, apparatus or article of manufacture using standard programmingor engineering techniques to produce software, firmware, hardware, orany combination thereof. The term “article of manufacture” as usedherein refers to code or logic implemented in hardware or computerreadable media such as optical storage devices, and volatile ornon-volatile memory devices. Such hardware may include, but is notlimited to, field programmable gate arrays (“FPGAs”),application-specific integrated circuits (“ASICs”), complex programmablelogic devices (“CPLDs”), programmable logic arrays (“PLAs”),microprocessors, or other similar processing devices.

Referring to figures, wherein like parts are designated with the samereference numerals and symbols, FIG. 2 is a block diagram illustrating astaggered backup-battery charging system 110 including a control device112 having a power sensor 114, a lower threshold 116, and an upperthreshold 118. The staggered backup-battery charging system 110 alsoincludes a charging circuit 120, a discharge circuit 122, and aplurality of battery packs 124. Each battery pack 124 includes one ormore batteries 126. The power sensor 114 measures the energy stored ineach battery pack 124 using a voltage sensor, a current sensor, or acombination of the two, and may include a test load. The chargingcircuit 120 may include a power source, a relay, and a current limitingdevice. The discharge circuit 122 may include its own relay and anothercurrent limiting device.

The lower threshold 116 establishes of a minimum acceptable effectiveenergy capacity and each battery pack 124 is recharged if its effectiveenergy capacity falls below the lower threshold. The upper threshold 118is representative of the minimum acceptable effective energy capacity ofthe lower threshold 116 plus a performance margin. In one exemplaryembodiment of the invention, the lower threshold 116 is set at 60% of abattery pack's initial maximum energy capacity and the upper threshold118 is set at 80% of the battery pack's initial maximum energy capacity.

The upper threshold 118 may be chosen to represent an energy capacityachieved through self-discharge in one half the time a battery pack willself-discharge to the lower threshold 116. However, in practice, theupper threshold is a function of several variables such as temperatureand individual variances in self-discharge rates of different batterypacks. Additionally, temperature effects can be accounted for bymonitoring the environment and adjusting the nominal self-discharge rateaccording to tables or equations. The effects of the variations betweenself-discharge rates of different battery packs can be adjusted bymonitoring the actual self-discharge of each battery pack versus timeand adjusting the tables or equations accordingly.

FIG. 3 is a block diagram of one embodiment of the control device 112 ofFIG. 2, wherein the control device 112 includes a processing device 128and a memory device 130. The memory device 130 includes memory locations132 holding the lower and upper thresholds 116,118.

The processing device may be a micro-processor, a general purposecentral processing unit (“CPU”), or programmable device such as an FPGA,ASIC, PLA, or CPLD. The memory device may a random-access-memory (“RAM”)device, a Flash RAM, or set of registers within the processing device128. In this embodiment of the invention, the control device 112 isresponsible for controlling the power sensor 114, the charging circuit120, and the discharge circuit. Additionally, the control device 112compares information from the power sensor 114 to the lower and upperthresholds 116,118.

FIG. 4 is a block diagram of another embodiment of the control device112 of FIG. 2, wherein the control device 112 includes a first hardwaredevice 134 for maintaining the lower threshold and a second hardwaredevice 136 for maintaining the upper threshold. The first and secondhardware devices 134,136 may be a voltage divider circuit or a voltagedetector. In this embodiment of the invention, output from the powersensor is applied to the first and second hardware devices 134,136. Thecontrol device reacts to signals from the first and second hardwaredevices 134,136 to direct the behavior of the charging circuit 120 andthe discharge circuit. The control device of this embodiment may itselfbe a hardware device such as a solid-state circuit.

In a two battery-pack system, if both battery packs fall below the upperthreshold (80%), the battery pack with the least effective energycapacity is recharged to the battery pack's maximum energy capacity. Inthis way, both battery packs are prevented from approaching the minimumacceptable effective energy capacity (60%) at the same time. Thisreduces the size or number of battery packs and reduces their associatedcost and volume. In order to maximize battery life and reduce thedeterioration of a battery pack's maximum energy capacity, the batterypack may be discharged to the minimum acceptable effective energycapacity before being recharged. The need to discharge before chargingis a function of the battery type and where the threshold is set. Forsome battery technologies and thresholds, discharging the battery beforecharging improves battery life and capacity. In other cases, there is nobenefit to the discharging, and so this step may be omitted.

The invention can be extended to a backup battery system have three ormore battery packs. If two or more battery packs fall below the upperthreshold, the battery pack with the least effective energy capacity isdischarged and then recharged to maximum energy capacity. Alternatively,the upper threshold may be set so that a battery pack is discharged andrecharged only when all the battery packs fall below the upperthreshold. In yet another embodiment of the invention, the upperthreshold may be set at the point an average battery pack wouldself-discharge to the lower threshold plus one-third of the differencebetween the lower threshold and the current maximum energy capacity.

In still another embodiment of the invention, the upper threshold may beset at the lower threshold plus two-thirds of the difference between thelower threshold and the current maximum energy capacity and anintermediary threshold may be set at the lower threshold plus one-thirdof the difference between the lower threshold and the current maximumenergy capacity.

The process of staggered backup-battery charging is illustrated by thestaggered charging algorithm 200 of FIG. 5. In step 202, each batterypack 124 is estimated by the power sensor 114 to ascertain its effectiveenergy capacity. The power sensor includes a battery capacity estimatorthat considers battery cell voltage, temperature history, the energyrequired for prior recharging, and electric current used when on load.Additionally, the power sensor may take into account manufacturer's dataon the battery cells. In this way, the battery capacity estimator woulduse the manufacturer's self-discharge curves versus time and temperatureas well as the manufacturer's maximum capacity versus time andtemperature curves in conjunction with a timer and periodic temperaturesampling to estimate the capacity.

The controller 112 compares these estimates to the upper threshold 118in step 204. If the effective energy capacities of all the battery packs124 are above the upper threshold 118, no action is needed and theprocess returns to step 202 to restart the cycle.

If the effective energy capacity of any of the battery packs 124 isbelow the upper threshold 118, the process proceeds to step 208. In step208, a determination is made as to whether the effective capacities ofall of the battery packs 124 are below the upper threshold 118. If so,the process proceeds to step 212 where the controller 112 directs thecharging circuit to charge the battery with the least amount ofeffective energy to its current maximum energy capacity. If at least onebattery pack is above the upper threshold, the process proceeds to step210.

In step 210, the effective energy capacity of the battery packs 124 arecompared to lower threshold 116. If any of the batteries packs have acapacity less than the lower threshold, the process proceeds to step 212where the controller 112 directs the charging circuit to charge thebattery packs with the least amount of effective energy to its currentmaximum energy capacity. If the effective energy capacities of all thebattery packs 124 are above the lower threshold 116, no further actionis needed and the process returns to step 202 to restart the cycle.

In step 212, the process of charging the battery may optionally includedischarging the battery to the lower threshold 116 using the dischargecircuit 122. After charging the battery pack with the least capacity,the process returns to step 202 to restart the cycle.

Those skilled in the art of making backup battery systems may developother embodiments of the present invention. For example, the inventionmay be extended to a backup-battery system including three or morebattery packs. If two or more battery packs fall below the upperthreshold, the battery pack with the least effective energy capacity isoptionally discharged to the lower threshold and then recharged to itscurrent maximum energy capacity. Alternatively, the upper threshold maybe set so that a battery pack is discharged to the lower threshold andrecharged only when all the battery packs fall below the upperthreshold.

However, the terms and expressions which have been employed in theforegoing specification are used therein as terms of description and notof limitation, and there is no intention in the use of such terms andexpressions of excluding equivalents of the features shown and describedor portions thereof, it being recognized that the scope of the inventionis defined and limited only by the claims which follow.

1. An article of manufacture including a data storage medium, said datastorage medium including a set of machine-readable instructions that areexecutable by a processing device to implement an algorithm, saidalgorithm comprising the steps of: detecting when a first energycapacity of a first battery pack falls below a lower threshold;comparing the first energy capacity to a second energy capacity of asecond battery pack; and, if the first energy capacity is less than thesecond energy capacity, directing a charging circuit to charge the firstbattery pack to a first maximum energy capacity.
 2. The article ofmanufacture of claim 1, further comprising the steps of: detecting whenthe first energy capacity and the second energy capacity fall below anupper threshold; and charging the first battery pack to the firstmaximum energy capacity.
 3. The article of manufacture of claim 2,further comprising the step of discharging the first battery pack to thelower threshold before the step of charging the first battery pack tothe first maximum energy capacity.
 4. The article of manufacture ofclaim 2, further comprising the steps of: detecting a third energycapacity of a third battery pack; detecting when any two of the energycapacities have fallen below the upper threshold; and charging the firstbattery pack to the first maximum energy capacity.
 5. The article ofmanufacture of claim 2, further comprising the steps of: detecting athird energy capacity of a third battery pack; detecting when the first,second, and third energy capacities have fallen below the upperthreshold; and charging the first battery pack to the first maximumenergy capacity.
 6. The article of manufacture of claim 4, furthercomprising the step of discharging the first battery pack to the lowerthreshold before the step of charging the first battery pack to thefirst maximum energy capacity.
 7. The article of manufacture of claim 5,further comprising the step of discharging the first battery pack to thelower threshold before the step of charging the first battery pack tothe first maximum energy capacity.
 8. A method of providing a servicefor managing a support system, comprising integrating computer-readablecode into a computing system, wherein the computer-readable code incombination with the computing system is capable of performing thefollowing steps: detecting when a first energy capacity of a firstbattery pack falls below a lower threshold; comparing the first energycapacity to a second energy capacity of a second battery pack; and, ifthe first energy capacity is less than the second energy capacity,directing a charging circuit to charge the first battery pack to a firstmaximum energy capacity.
 9. The method of claim 8, further comprisingthe steps of: detecting when the first energy capacity and the secondenergy capacity fall below an upper threshold; and charging the firstbattery pack to the first maximum energy capacity.
 10. The method ofclaim 9, further comprising the step of discharging the first batterypack to the lower threshold before the step of charging the firstbattery pack to the first maximum energy capacity.
 11. The method ofclaim 9, further comprising the steps of: detecting a third energycapacity of a third battery pack; detecting when any two of the energycapacities have fallen below the upper threshold; and charging the firstbattery pack to the first maximum energy capacity.
 12. The method ofclaim 9, further comprising the steps of: detecting a third energycapacity of a third battery pack; detecting when the first, second, andthird energy capacities have fallen below the upper threshold; andcharging the first battery pack to the first maximum energy capacity.13. The method of claim 11, further comprising the step of dischargingthe first battery pack to the lower threshold before the step ofcharging the first battery pack to the first maximum energy capacity.14. The method of claim 12, further comprising the step of dischargingthe first battery pack to the lower threshold before the step ofcharging the first battery pack to the first maximum energy capacity.